Spectral and relative intensity noise (RIN) characteristics of a single-frequency Raman fiber amplifier co-pumped by amplified spontaneous emission (ASE) sources are investigated experimentally. Due to the relatively lower intensity noise of ASE sources compared to usual fiber laser pumps, the full width at half maximum (FWHM) linewidth of the signal laser increases negligibly. But there is significant increase in RIN and spectral wings due to the noise transfer at high frequency from the ASE source during the Raman amplification. The deterioration can be suppressed to some extent with ASE of broader linewidth, which has lower intensity noise.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Raman fiber lasers (RFL) have attracted great interest in the past decades for wider wavelength coverage compared with rare-earth-doped fiber lasers [1,2]. Providing appropriate pump laser, Raman laser can be generated at arbitrary wavelength within the transmission window of optical fibers. Kilowatt Raman fiber lasers were reported with standard Yb-doped fiber laser or amplifier as pump sources [3–5]. However, these high power RFLs have broad spectrum, which are not suitable for many applications requiring further nonlinear frequency conversion. For instance, single-frequency or narrow-linewidth Raman fiber amplifiers (RFAs) operating at 1.1∼1.2 µm are frequency doubled to generate yellow-orange-green lasers , which have applications in medical science, biological imaging and astronomy . In particular, single-frequency RFA at 1178 nm, after frequency doubling to 589 nm, is used in sodium laser guide star [7–9]. In high-power single-frequency RFAs, the stimulated Brillouin scattering (SBS) is the main power-limiting factor. Several methods for suppressing SBS, such as acoustically tailored fiber , longitudinally varied strain  and thermal gradients along the gain fiber , etc., have been validated.
Meanwhile, third-order optical nonlinearity may broaden the laser linewidth during the Raman amplification. Previous experimental and theoretical works have shown that pumping configuration and the temporal property of pump laser determine the spectral behavior of single-frequency RFAs [12–15]. In counter-pumped RFAs, due to the low-pass filter effect resulting from the pump-signal light walk-off, the high frequency temporal fluctuation of the pump laser would not be transferred to the signal laser. Linewidth broadening is negligible . Nevertheless, high-power wavelength division multiplexers (WDMs) are needed to combine and separate pump and signal lasers. In particular, the output WDM with extra delivery fiber complicates the SBS suppression, and limits the achievable narrow linewidth output [2,8,9]. In co-pumped configuration, the output WDM is unnecessary which can be replaced by a free space dichroic mirror. But the intensity noise of the pump laser would be easily transferred to the signal laser and resulting in the linewidth broadening. It is straightforward to realize that the linewidth broadening issue may be overcome using stable pump laser. In , an ultra-low noise pump laser was developed with a low noise phase-modulated single-frequency seed laser and two stages of power amplifiers, and linewidth-maintaining Raman fiber amplification was demonstrated in co-pumping configuration. But the low-noise pump laser in this demonstration is complicated and of high cost. Amplified spontaneous emission (ASE) also has good temporal stability because of its broad and longitudinal-mode-free spectrum [16,17]. Recently, ASE was proven to be promising pump source for various RFLs [13,18,19]. Is ASE source also a candidate for co-pumping single-frequency RFA?
In this paper, we experimentally investigate the spectral and RIN characteristics of a single-frequency RFA co-pumped by ASE sources. For comparison, ASE sources with two different full width at half maximum (FWHM) are used as pump laser. A RIN comparison shows that the ASE source with the FWHM linewidth of 10 nm is quieter than that of 3 nm. The 3-dB spectral linewidth of the single-frequency RFA retains in the range of experimental errors with both ASE sources. But the spectral wings rise with power, although it is reduced when the quieter 10 nm ASE source is used as pump. In addition, the RIN at high frequency increases with output power due to the noise transfer from the ASE pump, which is greater than the RIN on the pump source.
2. Experimental setup
The experimental setup is depicted in Fig. 1. The seed laser is a single frequency diode laser with a central wavelength of 1122 nm and an output power of 10 mW. The seed laser is delivered into a co-pumped RFA after a standard circulator (Circulator 1). Port 3 of Circulator 1 is employed to detect the backward light and monitor the SBS effect. The pump laser has a standard master oscillator power amplification (MOPA) configuration using all polarization-maintaining (PM) active and passive fibers which is similar with our previous reported linearly-polarized broadband ASE source in . The central wavelength and FWHM linewidth of the ASE pump source are decided by the filters in the setup. Filter 1 have a bandwidth of either 3 nm or 10 nm, and centered at 1065.0 nm. Filter 2 has a bandwidth of 10 nm. All the three amplifier stages use single-mode Yb-doped fiber with a core diameter of ∼10 µm and a numerical aperture of 0.075 as gain medium, which has a nominal 4.8 dB/m cladding absorption for the 976 nm diode pump. The output from the ASE source is optically isolated (ISO 5) and injected into the Raman gain fiber after WDM 1. The power of the ASE source is limited to 20 W to avoid the damage of ISO 5. The Raman gain fiber used is a standard polarization-maintained single mode fiber (PM980-XP, Nufern Inc.) of 110 m long. The residual pump laser is stripped out by WDM 2. All the fiber ends are either angle cleaved or spliced to patch cord with angled end face to avoid back reflections.
3. Results and discussion
Firstly, the stability of the ASE pump source is investigated in detail. Previous research showed that in ASE sources pumped Raman amplifiers, the spectral broadening of Stokes wave is linewidth dependent . Therefore, in the experiments ASE sources with two different linewidths are examined. The output spectra at different powers are measured and shown in Fig. 2. With two filters of different bandwidth (Filter 1@3 nm, Filter 2@10 nm), step-shaped spectra are observed as shown in Fig. 2(a). The spectrum broadens at higher power due to fiber optical nonlinearity, but which only influences the wing of the spectrum. The 3-dB linewidth of 3 nm retains at higher powers. When two 10-nm-bandwidth filters are used, the spectral broadening of the ASE source is negligible. The observation indicates a quieter output from the ASE source with 10-nm linewidth than that of 3-nm indeed.
For a complete characterization of the intensity stability, RIN spectra of the ASE sources are measured at different output power and shown in Fig. 3(a) and (b) for the cases of 3 nm and 10 nm linewidth, respectively. The intensity noise is measured with a high-speed and low-noise InGaAs photoelectric detector with a bandwidth of 1 GHz. The RIN characteristic in a Fourier frequency range from 100 Hz to 1 GHz is displayed in Fig. 3. Since there is no vibration and sound insulation for the laser, the environment noise is transferred to the laser, resulting in many discrete sharp peaks in the low frequency range (< 2 kHz) of the RIN spectrum. As shown in Fig. 3, aside from the environmental noise spikes, the RIN at low frequency (<100 kHz) decreases obviously when the output power is scaled from 5 W to 20 W because of pump noise reduction and gain saturation . At high frequency range (> 1 MHz), the RIN spectra become flat around −124 dBc/Hz and −127 dBc/Hz for the cases of 3 nm and 10 nm, respectively. The sudden drop after 500 MHz is due to the bandwidth limitation of the detection setup. So, the ASE source with 10 nm linewidth is apparently more stable than that with 3 nm.
After full characterization, the ASE sources are used to co-pump the RFA as shown in Fig. 1. The RFA output powers versus pump powers are measured and shown in Fig. 4. The power curves almost overlap with each other. With 20 W pump power of 3-nm-ASE and 10-nm-ASE source, the output power is 1303 mW and 1296 mW, respectively. The backward light from port 3 of the circulator does not exceed 1 mW at full power, because the RFA is below SBS threshold. For the RFA co-pumped with ASE source, the independence of optical efficiency on the pump spectral linewidth is observed also in .
The output spectra of the RFAs are measured firstly with an optical spectrum analyzer. It cannot resolve the FWHM linewidth, but will give information on the wide optical spectrum. Figure 5 illustrates the output spectra of different ASE-pumped RFAs at pump power of 0, 5, 10, 15 and 20 W. Both RFAs have increasing spectral background at higher power, but the RFA pumped with 3-nm ASE increases faster. It has a 4 dB higher spectral background at 1 nm away from the center wavelength than that of 10-nm ASE pumped RFA at the power of 1.3 W.
The spectral difference and evolution can be more clearly characterized with the root-mean-square (RMS) linewidth . Figure 6(a) and (b) show the 3, 5, 10, 15 and 20-dB RMS linewidth of RFAs versus the pump power. The linewidth at the pump power of 0 W corresponds to the relevant value of the single-frequency seed. For both cases, the linewidths increase with powers. However, the RFA pumped with 10-nm ASE increases much slower, and nearly unchanged at 3-dB and 5-dB. That’s because the 10-nm ASE has lower intensity noise as discussed in the previous paragraph.
To resolve the spectrum of the single frequency diode laser and RFA, other methods should be applied. We use the delayed self-heterodyne technique to measure the linewidth . The resolution of the measurement is determined by the delay fiber length, which is 9.5 km in the measurement setup. When the delay time is over six times of the laser coherence time, the measurement is accurate enough . So the setup can accurately measure linewidth of about 100 kHz.
Firstly, ASE source with 3-nm linewidth is used as pump laser, the linewidth measurement results of the RFA at different power are shown in Fig. 7(a). The laser is frequency-shifted by an acousto-optic modular (AOM) operating at a frequency of 150 MHz. From Fig. 7(a), it can be found that the signal-to-noise ratio (SNR) reduced from 43 dB for the seed to 26 dB for the maximum power. Similar increase at spectral wings is observed in optical spectrum measurement as shown in Fig. 5(a). But the spectral linewidth of the Raman amplified signal laser is hardly broadened with power scaling. When the pump laser of the RFA is changed to 10-nm ASE source, the rise at the spectral wings is noticeably reduced. SNR is 32 dB at maximum power as compared to 26 dB in the 3-nm pump case. The FWHM of the signal again keeps constant with increasing power. Thus, at least with the more stable 10 nm source, the linewidth broadening is under control in co-pumped RFA.
The rise of spectral wings, but with negligible FWHM linewidth broadening, suggests that the RFA output has significant frequency noise at high frequencies . These high frequency noises are transferred during the Raman amplification from the ASE pump sources. The RIN spectra of the RFA output at different pump powers are measured and shown in Fig. 8. Also shown in Fig. 8 are the noise floor of the setup and the RIN spectrum of the seed laser. The RIN of the seed laser is about −150 dBc/Hz at frequency higher than 10 kHz. When pumped with the 3 nm ASE source, the RIN of RFA at 1 MHz increases to −119 dBc/Hz and −107 dBc/Hz, when the pump power is 5 and 20 W (output 29 mW and 1303 mW), respectively. Note the 3-nm ASE source has RIN value of −124 dBc/Hz at 20 W. When pumped with the 10-nm ASE source of lower RIN (−127 dBc/Hz), the RIN of RFA increases to −125 dBc/Hz and −114 dBc/Hz at the pump power of 5 and 20 W (output 34 mW and 1296 mW). The RIN of RFA at 20 W pump is 7 dB lower than the 3 nm case. Nevertheless, for both cases, the RIN of RFAs is significantly higher than that of corresponding pump sources. The observation is consistent with the previous studies on pump to signal RIN transfer in Raman fiber amplifiers, for example Ref. . In co-pumped high gain RFA, the signal RIN is always higher than the pump RIN by an amount depending on the amplifier gain.
Therefore, although the linewidth broadening can be negligible in co-pumped RFA with ASE sources, the RIN deterioration is a severe issue if the amplifier is intended for applications with high RIN requirement. The ASE sources have relatively high and flat noise at high Fourier frequency. For comparison, the RIN spectra of counter-pumped RFA with the 10-nm ASE source are measured and shown in Fig. 9. Characteristic low-pass filtered RIN spectra with periodic dips are observed . The frequency of the first dip corresponds to the round-trip time of the amplifier length, which is 932 kHz in the experiment. At frequency lower than a few 100 kHz, the RIN is comparable to the co-pumping case. However, at higher frequency the RIN drops quickly and down to −148 dBc/Hz over 100 MHz, which is significantly lower than that of co-pumped case.
The RIN and spectral properties of single frequency RFA co-pumped with ASE sources are investigated experimentally in detail. The results prove that the linewidth broadening can be controlled with ASE pumping, because ASE sources have overall lower intensity noise than common fiber lasers. However, although 3 dB linewidth broadening is negligible, the spectral wing rises, and the RIN at high frequency increases with output power. It is due to the noise transferred from the ASE pump, which increases both the frequency noise and RIN of the RFA output. The RIN on the signal laser after the Raman amplifier is greater than the RIN on the pump source. Since the intensity noise of ASE improves with broader bandwidth, a lower RIN and spectral broadening is observed for 10 nm ASE pumping than 3 nm source. Thus, the spectral and RIN properties may be improved by further optimizing the ASE source. However, we think in this stage that ASE co-pumped single frequency RFA is not suitable for applications which has tight demand on RIN and spectral purity. As for counter-pumped single-frequency RFA, the walk-off effect between pump and signal laser would form as a low-pass filter in the RIN spectrum, which partially filter the noise at high Fourier frequency.
National Key Research and Development Program of China (2020YFB04012600, 2020YFB1805900); National Natural Science Foundation of China (62075226).
The authors declare that there are no conflicts of interest related to this article.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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